These studies are designed to explore the control of gene expression during the pivotal post-division (G1 or GO) stage of the cell cycle. An inducible bifurcating developmental system is described in which a homogeneous population of quiescent (GO) cells may be directed to either enter the division cycle or to differentiate without division. Both responses are synchronous and occur within the same time frame. This system will be used, in conjunction with a double labelling strategy and fractionation (both cellular and molecular) technques, to detect molecular events specific to each developmental response, in an effort to identify the controlling influences on differential gene expression. Special attention will be given to non-histone nuclear proteins and DNA binding proteins. A parallel developmental system is described. In this, two different homogeneous populations of cells, quiescent GO and arrested synchronous post-division G1 cells, are induced to enter the division cycle, both synchronously, but with different time scales. A comparison will be made of the "commitment" step and of surface transport properties following the induction to proliferate. The nature of the transition of G1 to GO will also be explored. Changes in the structure of G1 or GO cells as a population passes from logarithmic growth to stationary growth will be monitored cytokinetically with the use of two parameters which permit quantitative manipulation in a manner analogous to the Michaelis-Menten analysis of enzyme kinetics. These studies will be complemented by analyses of mutants selected for temperature sensitive lesions in the response to induction to proliferate or differentiate.